glSampleCoverage — specify mask to modify multisampled pixel fragments
void fsfuncglSampleCoverage( | GLclampf value, |
GLboolean invert); |
void fsfuncglSampleCoveragex( | GLclampx value, |
GLboolean invert); |
value
Specifies the coverage of the modification mask. The value is clamped to the range [0, 1], where 0 represents no coverage and 1 full coverage. The initial value is 1.
invert
Specifies whether the modification mask implied by
value is inverted or not.
The initial value is GL_FALSE.
glSampleCoverage defines a mask to modify
the coverage of multisampled pixel fragments. This capability is used
for antialiased screen-door transparency and smooth transitions between
two renderings of an object (often for level-of-detail management in
simulation systems).
When multisampling is enabled (see
glEnable
with argument
GL_MULTISAMPLE) a ``fragment mask'' is computed for
each fragment
generated by a primitive. This mask reflects the amount of the pixel
covered by the fragment, and determines the frame buffer samples that may
be affected by the fragment.
If conversion of alpha values to masks is enabled
(glEnable
with argument
GL_SAMPLE_ALPHA_TO_COVERAGE),
the fragment alpha value is used to
generate a temporary modification mask which is then ANDed with the
fragment mask. One way to interpret this is as a form of dithering: a
multivalued alpha (coverage or opacity) for the whole fragment is
converted to simple binary values of coverage at many locations (the
samples).
After conversion of alpha values to masks, if replacement of alpha values
is enabled
(glEnable
with argument
GL_SAMPLE_ALPHA_TO_ONE), the
fragment's alpha is set to the maximum allowable value.
Finally, if fragment mask modification is enabled
(glEnable
with argument
GL_SAMPLE_COVERAGE),
glSampleCoverage defines an additional modification
mask. value is used to generate a modification mask in much the same way
alpha was used above. If invert is GL_TRUE,
then the modification mask
specified by value will be inverted. The final modification mask will
then be ANDed with the fragment mask resulting from the previous steps.
This can be viewed as an ``override'' control that selectively fades the
effects of multisampled fragments.
Note that
glSampleCoverage(value, GL_TRUE)
is not necessarily equivalent to
glSampleCoverage(1.0 - value, GL_FALSE);
due to round-off and other
issues, complementing the coverage will not necessarily yield an inverted
modification mask.
glGet with argument GL_SAMPLE_COVERAGE_VALUE
glGet with argument GL_SAMPLE_COVERAGE_INVERT
glIsEnabled with argument GL_SAMPLE_ALPHA_TO_COVERAGE
glIsEnabled with argument GL_SAMPLE_COVERAGE
Copyright © 2003-2004 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.